The present invention relates to an overcurrent relay used to prevent heat damage of an electric motor etc. due to the overload thereof, and in particular, to a mechanism for adjusting the operating current in an overcurrent relay.
FIGS. 1 and 2 show respectively a rear elevation and a side view of a conventional overcurrent relay. The overcurrent relay comprises a plastic casing 1 having a back opening portion and a plastic cover 2 closing the back opening portion of the casing 1. In the upper portion of the casing 1, there are disposed a plastic adjusting dial 3 for adjusting the operating current of the overcurrent relay, and an adjusting screw 4 screwed into the casing 1. The adjusting dial 3 is attached to the top of the adjusting screw 4. A belleville spring 5 is disposed between the casing 1 and the head of the adjusting screw 4 to prevent the screw 4 from unintentionally rotating and unnecessarily moving due to the elastic force of the spring. Each of terminals 7 is attached to the cover 2 to electrically connect the overcurrent relay to a main circuit of a well known electromagnetic contactor.
In FIG. 3 for explaining the operation of portions of the overcurrent relay, each of heaters 71 is electrically connected to a terminal 7 and generates heat due to the electric current flowing through the main circuit of the electromagnetic contactor. The ends of the heaters 71 are respectively secured to terminals 72 and 73. Each bimetal 74 is juxtaposed in a facing relationship to a respective one of the heaters 71, and is secured at one end thereof to each terminal 72 and disposed at the other end thereof within a recessed portion 75a of an actuating plate 75 with a predetermined clearance therebetween for idle. When a normal current flows through the heater 71, the end of each bimetal 74 is located within each recessed portion 75a. Since the actuating plate 75 can be in contact with the other end of each bimetal 74, a deflection of the bimetal 74 can be transmitted to a temperature compensating bimetal 76 which is connected at a lower portion thereof to the actuating plate 75 and at the upper portion thereof to an operating lever 77. The operating lever 77 is rotatably supported by a rotary shaft 78 which is supported by an adjusting member 79. The adjusting member 79 is rotatably supported at a corner thereof by a support member 80 disposed in the casing 1, and is engaged along an upper surface thereof with the lower end of the adjusting screw 4.
A normally closed contact disposed near the adjusting member 79 comprises a stationary contact element 87 and a movable contact element 81 which is movable with respect to the stationary contact element 87. The movable contact element 81 is secured to an insulated plate 82 which is rotatably supported by a support member 83 at a fulcrum 84 thereof. A spring support member 85 is attached to a lower portion of the support member 83. A tension spring 86 is connected at one end thereof to the spring support member 85 and at the other end thereof to the movable contact element 81 of the normally closed contact. The operating member 77 can contact the tension spring 86 by the rotation of the operating member 77 around the rotary shaft 78. A normally open contact is disposed near the support member 83 and comprises a stationary contact element 89 and a movable contact element 88 which is made of a thin elastic metallic plate and movable with respect to the stationary contact element 89. The movable contact element 88 can be moved towards or away from the stationary contact element 89 by the lower portion of the insulated plate 82.
In the overcurrent relay constructed as described above, each bimetal 74 is heated and deflected by each heater 71 through which an electric current flows. When a normal electric current flows through each heater 71, the end of each bimetal 74 stays within each recessed portion 75a of the actuating plate 75 without pressing on the actuating plate 75. However, when an excessive electric current flows through either one of heaters 71, the bimetal 74 is deflected so that the lower end of the bimetal 74 engages the actuating plate 75 and moves it leftwards in FIG. 3. The temperature compensating bimetal 76 moving leftwards causes the operating lever 77 to rotate around the rotary shaft 78 in the clockwise direction and moves the tension spring 86 leftwards. When the tension spring 86 is moved leftwards by the operating lever 77 beyond its dead point, the movable contact element 81 is rapidly rotated around the fulcrum 84 in the counterclockwise direction. The upper portion of the insulated plate 82 then contacts a reset bar 6 which stops the insulated plate 82 from further rotation. Thus the normally closed contact opens due to the movable contact element 81 moving away from the stationary contact element 87, and simultaneously the normally open contact is closed, since the movable contact element 88 is moved towards and contacts the stationary contact element 89 as a result of the lower portion of the insulated plate 82 pressing the movable contact 88 in the right direction in the FIG. 3. Accordingly, when the normally closed contact is electrically connected in series to an electric circuit having a coil of an electromagnet contactor for driving and keeping a contact thereof in a closed state, and the normally open contact is electrically connected to an annunciator such as an alarm whistle or an alarm lamp, at the time of an overload of a motor connected to the electromagnetic contactor, the main circuit can be interrupted to prevent damage from occurring to the motor and an alarm for indicating the overloading condition can be activated.
Thereafter, when the state of the motor is changed from the overload state to the normal state and each bimetal 74 is returned to its original position, the reset bar 6 is pushed down to return the movable contact elements 81 and 88 to their original positions. Thus the normally closed and open contacts are respectively closed and open.
When the adjusting dial 3, which is mounted on the head of the screw 4 for rotation therewith, is rotated, the screw 4 is axially moved and the adjusting member 79 rotates with respect to the support member 80 so that the position of the rotary shaft 78 is moved either to the left or to the right in FIG. 3, thereby adjusting the operating current of the overcurrent relay.
The undesirable movement and rotation of the adjusting screw 4 are prevented by the elastic force of the belleville spring 5. When the overcurrent relay is attached to an electromagnetic contactor and the contactor is repeatedly switched, the adjusting screw 4 may be unintentionally rotated due to vibration of the container or other devices, thereby changing the set operating current. Furthermore, the adjusting dial 3 may be separated from the adjusting screw 4 due to the vibrations.